Conversion of wave motion into electrical energy



April 1941- B. BAUMZWEIGER 2.237298 CONVERSION OF WAVE MOTION INTQ ELECTRICAL ENERGY Filed Sept. 29, 1958 4 Sheet-Sheet l ,wfi d .27'\ 1500 z Z a; 2 23 -2 I d-Zfg April 1941- B. BAUMZWEIGER .237.298

CONVERSION OF WAVE MOTION INTO ELECTRICAL ENERGY 4 Sheets-Sheet 2 2 Filed Sept. 29, 1938 1,000 i v 10 00 flefuerzcy fyczes ver' C ecarzd fipeflzor'l e72 amznj azlmzwe April 8, 1941. B. BAUMZWEIGER CONVERSION OF WAVE MOTION INTO ELEC'IfRICAL ENERGY- Filed Sept. 29, 1938 4 Sheets-Sheet 3 fivenz r, P5872 amZ njaFzem /e 37*,

April 1941- B. BAUMZWEIGER 2,237,298

CONVERSION OF WAVE MOTION INTO ELECTRICAL ENERGY Filed Sept. 29, 1938 4 Sheets-Sheet 4 fizvenrz' I I wager 6 5596 Patented Apr. 8, 1941 CONVERSION OF WAVE MOTION INTO ELECTRICAL ENERGY Benjamin Baumzweiger, Chicago, 111., now by change of name Benjamin B. Bauer, asslgnor to S. N. Share and Frances Shure, trustees, doing business as Shure Brothers, a. partnership Application September 29, 1938'. Serial No. 232,439

18 Claims.

transducers through a combination a: a unit having a nondirectional (circular) polar sensitivity pattern with one having a bidirectional (cosine-law) polar sensitivity pattern. A combination of two such units causes the resulting polar sensitivity pattern" to be unidirectional- (cardioid) in shape, and it has been applied extensively in the past to transmitting antennas,-

microphone apparatus, etc. For this latter application, one of the units is commonly made to operate on the pressure component of the sound wave (pressure transducer) and the other upon the pressure-difference of the sound wave (velocity transducer). Addition or cancellation of the voltages generated in each unit occurs depending upon whether the incidence of sound is from the front (0 incidence) or from the rear (180 incidence) of the instrument. Obviously, the voltages generated by both units for the 180 incidence should be substantially equal and opposite in phase throughout the frequency range in which the cancellation s desired, which because of inherent differences in construction and operating principle is a difiicult thing to obtain in microphones operating upon dissimilar components of the sound wave.

One important object of my invention is to provide a unidirectional transducer operating over a wide frequency range and comprising in part two transducing elements operating on the same component of the sound wave, thus doing away with the necessity of subtracting outputs of two transducing elements working on dissimilar components of the sound wave.

Another object is to provide a unidirectional transducer with marked unidirectional properties over the operating range of frequencies.

alized apparatus embodying my invention; Fig.

2, a vector diagram showing the voltage relationships for a zero degree incidence of sound; Fig. 3, a similar view to Fig. 2 but representing the 180 incidence of sound; Fig. 4, a. diagrammatic view of a specific embodiment comprehended within the diagram of Fig. 1; Fig. 5, a polar diagram illustrating the directional characteristics of the transducer of Fig. 4; Fig. 6, a diagrammaticand sectional view of a unidirectional crystal microphone; Fig. 7, a rear view in elevation of the same; Fig. 8, an equivalent electrical circuit of the microphone shown in Fig. 6; Fig. 9, a frequency response curve of the microphone shown in Fig. 6, the upper curve showing the front side response and the lower dotted line showing the decrease of response for the rear incidence sound; Fig. 10, a part sectional view of a unidirectional dynamic microphone; Fig. 11, a front view of the same; Fig. 12, a diagrammatic view of the equivalent electrical circuit of the microphone shown in Fig. 10; Fig. 13, a cross-sectional view' of the unidirectional crystal microphone equipped with an acoustical resistance formed of cloth; Fig. 14, a front elevation of a unidirectional moving conductor microphone;Fig. 15, a sectional view, the section being taken as indicated at line iii of Fig. 14; Fig. 1.6, a sectional view, the section being taken as indicated at line It of Fig. 14'; and Fig. 17, a diagrammatic view of the equivalent electrical circuit of the microphone shown in Fig. 14.

My invention is principally applicable to production and reception of sound waves in air, although it will become apparent to those skilled in the art that it may be equally applicable to wave phenomena in other media. The transducer element or elements employed may be either of the reversible type. such as piezoelectric crystal, moving coil, moving armature or condenser type, or of the non-reversible type such as, for example, the carbon-type. The theory set forth herein is applicable to receiving apparatus, such as loudspeakers, as well as to transmitting apparatus such as microphones. If transducers of the reversible type are employed, one instrument could serve interchangeably, both as a transmitter and ass. receiver.

The nature of my invention is such that it can be best explained by reference to the following equivalent electrical networks and circuit equations. Fig. 1 is a schematic representation" of two electroacoustic transducers A and A, generating respectively voltages E and E, and the interconnecting electrical network (I. The transducers, which may operate on any function of the sound wave whatsoever, are spaced by an effective acoustical distance d which in general should be smaller than, or comparable to, onequarter wavelength of the highest frequency at which unidirectional action is desired, although it will be shown later that transducers may be constructed having unidirectional. properties at frequencies higher than that specified above'by virtue of diffraction and other wave eflects. C is a generalized network shown in an equivalent 1r section, composed of impedances Z0, Z1, Z2, and Z, Z1, and Z2. The impedance Z: is connected to the receiver B which may be an amplifier or any other receiving device. For simplicity, the internal impedances of the transducers A and A are here considered negligible, although if this sponding arrow in Fig. 1. The voltage developed by the transducers A and A is indicated as E and E respectively. Subscripts (0), (0) and (180) are used to designate voltages developed for any angle of incidence 0, for normal front (0) or for the rear (180) incidence of sound, respectively.

The respective voltages generated by the transducers A and A will be displaced in phase by an angle given by the equation:

ii cos 0 (I) in which qt; is the phase angle between the voltages E and E w is the expression 21rf f is the frequency, cycles per second 0 is the angle of incidence of sound Cv is the velocity of the sound wave Applying circuit analysis to the equivalent circuit of Fig. 1, it may be shown that the voltage e delivered to the receiving apparatus, is given by the equation:

It may be shown, furthermore, that the voltage drop across any branch in anetwork composed of linear elements, due to the action of two sources A and B connected at any two points, may always be expressed as the sum of the partial voltage drops due to each source acting alone. Thus, for the network of Fig. 1, the portion of e due to E can be expressed as PE where P is the coefficient of E in Equation II divided by the denominator, and the portion of e due to E can be expressed as QE where Q is the coefficient of E in Equation II divided by the denominator. It is seen therefore that the expression for the voltage delivered to the receiving apparatus can be indicated in the form:

Any expression involving network elements, having the function of P and Q in Equation III, is herein called the network factor.

To obtain unidirectional action, the voltage emu should become zero. Therefore, the condition to be met is:

and hence the relation between coefllcients P and Q should be such that:

and the nature of the network components is to be chosen to substantially maintain this relation throughout the frequency range in which the unidirectional action is desired.

Equation V is perfectly general and may be applied to any unidirectional transducing system having two generators and in interconnecting network, delivering the translated energy to a receiver. For the particular case of the network of Fig. 1, the values of network factors P and Q specified above may be inserted into, e. g., (V), giving the following relationship to be fulfilled:

Before describing more specific embodiments of my invention, its operation will be further clarified by the following'explanation made in reference to Figs. 2 and 3, which are vector diagrams representing the voltage relations for front and rear incidence of sound upon the instrument of Fig. 1.

For the purpose of explanation, it is assumed that voltages E and E, generated by the similar generators A and A, are of unequal magnitudes, although this is not necessarily the case. The voltage E0 is shown leading the voltage E'o through an angle determined by Equation I, while the voltage Em is showndagging behind E'iao by the same angle, since reversal of the direction of incidence brings about reversal of the relative phase positions of the generated voltages. The network factors P and Q are shown of the same relative magnitudes and angular position as the rear incidence voltages E'm and Em respectively, as specified by Equation V.

The 0 (front-incidence) condition is shown in Fig. 2. The voltage E0 is operated upon the vector P to give the vector PEn which is the contribution of the generator A to the total output voltage. The voltage E'o is operated upon by vector Q giving the vector QE' which is the contribution of the generator A to the total output voltage. QE'o is added to PE) giving the resultant output voltage e The (180") rear incidence condition is shown in Fig, 3. The voltage Em is operated upon by the vector P giving the vector PEllO which is the contribution of the generator A to the total output voltage. The voltage Em is operated upon by the vector -Q giving the vector QE'm which is the contribution of the generator A to the total output voltage. It should be noticed that for the rear incidence condition, the voltages PEiwand QE'm are out of phase and of equal magnitude, and hence when the latter is added to the former, the resulting total output voltage is zero.

A specific example of network selection will be given in reference to Fig. 4. This network is the same as that of Fig. 1, with the following element values:

It will be assumed here that the voltages E and E are two vectors of equal magnitude and displaced 'Jy an angle in whose value is determined from Equation I, their ratio being therefore equal to a unit vector at the angle Substituting this value of angle into the right hand side of Equation VI, and that of the network elements of (VII) into the left hand side of Equation VI, it is evident that the desiredrelationship isi 1-w L'C+jwC-R' Q g The left hand member of the Equation VIII represents a quotient of two vectors, each of which may be made very nearly a vector operating at an angle proportional tofrequency, if the relationship between resistance, inductance and capacitance is such that:

since substituting these values into Equation IX gives the following relation:

It should be observed that the numerator and the denominator of Equation X are the major terms of the expansion for the cosine and the sine functions: hence, as long as:

The frequency term or drops out of this equation, and therefore the condition for unidirectivity will be obtained if (XIII) The distance d and the velocity of sound Cv being known, R, R, C, and C may be selected by the use of Equation XIII. Then values of L and L' may be computed from Equation IX.

Since the Equation XII holds as long as the expressions (XI) are true, then by choice of sufficiently small distance d,'unidirectional action may be obtained throughout a wide range of frequencies. I have found that Equation XII is valid up to frequencies at which d is not larger than one-quarter the wavelength of sound; thus, if d is equal to approximately 1.5 cm., undirecaas'naos (VIII) angle t; therefore, the ratio of Em and E'iao will be a vector K at an angle qui therefore: ePE(K|g,K|M)=PEK(|@-|g (Xv The expression in parenthesis of Equation XV, at frequencies for which (1 is small compared to one-quarter wavelength of sound,- may be shown to approximately equal the algebraic sum of the angles as. and man. Substituting the values of these angles given by Equation I,

Parr-iguacos 0) (XVI) If the character of the transducers A and A is such that the voltage generated is independent of the incidence of sound (pressure-operated or non-directional transducers), the polar characteristic of the combination will be a cardioid of revolution expressed by the quantity in parenthesis in Equation XVI. This polar characteristic is shown graphically in solid line in Fig, 5.

tional action is obtained for all frequencies up Since A and A are similar generators, the ratio of voltages E and E will be a vector K having constant magnitude and acting at the If the voltage E and E varies as the cosine of the angle of incidence, which will occur if transducers are of the bidirectional or velocity-type, then E'=E'o cos 0 and:

The quantity in brackets of Equation XVI represents the polar characteristic shown graphically in dotted line in Fig. 5. It is seen, therefore, that combining two velocity-type transducers and the network described results in an electroacoustic transducing instrument of very marked unidirectional properties. It will be observed that my invention may make use of any two transducers operating on the same wave function, even if their transducing principles were dissimilar.

Instead of providing the electrical network directly at the output of the transducers, it is possible to first amplify these outputs with two independent amplifiers and combine the outputs after the amplification. This procedure would be considered of the nature of an equivalent.

Instead of employing two transducers and an electrical network to obtain unidirectional operation, my invention makes such operation possible through modifying wave disturbances at two points in space by means of equivalent acoustical networks and impressing these disturbances upon one electroacoustical transducer. An embodiment of my invention employing this alternative is shown in cross section and rear elevation in Figs. 6 and 7. The transducer assembly consists of a diaphragm 22 suitably supported in a casing 23 which also contains the piezoelectric crystal 24. The forces developed by sound phessure at the diaphragm are transmitted to the crystal by means of a connecting member 25, and the electrical energy developed therein is received from the crystal by means of conductors 41 and 48. The front side of the diaphragm is provided with an acoustical damping screen 26 constituted of a suitable wire-screen support having one or more thicknesses of cloth forming acoustical resistance and inertance. Between the diaphragm 22 and screen 26, there is a cavity 21 having an acoustical compliance C.

The casing 23 has a circular opening 28 which serves as a housing for the piezoelectric crystal and also forms part of the acoustical network. The housing 23, the back plate 29, and the diahragm 22 provide a cavity 40. At the rear side of the case by means of screws 4| is held a cover 29 provided with ridge 30, the proper spacing being the outside of the passage. 3|.

obtained by adjustment against compression of spring 42. Thus, a narrow passage 31 is formed, having acoustical resistance and an inertance. P denotes the sound pressure at the outside of the damping screen; -P' denotes the sound pressure at The effective acoustical path between these pressures is called d. I have found that, at frequencies of sound for which the diameter of the casing 23 is smaller than one-half wave-length, the pressures P and P are essentially'equal and separated by a phase angle given by Equation 1.

The equivalent electrical circuit of the transducer and its associated acoustical network appears in Fig. 8 in which R and L, and R and L' are the acoustical resistances and inertances of the screen 26 and the passage II, respectively; C and C are acoustical compliances of the cavities 21 and 40 respectively. Z3 is the impedance of the transducer element itself. As a simplifying assumption, the impedance Z: is considered as formed by the capacitance C: corresponding to the stiffness of the crystal 24, and the reactions vof the medium are neglected. The voltage e developed across Z3 represents the resultant pressure upon the piezoelectric crystal. It may be observed that this equivalent circuit is entirely identical with that of Fig. 4,, and therefore all of the equations derived previously may be applied to it. Thence, the acoustic capacitance, resistance and inertance terms are selected in reference to Equations IX and K111 to provide the unidirectional action desired. The terms R, L, and C due to screen 26 and cavity 21 are small compared with terms R, L and due to passage 3| and cavity 40, hence the last term of the right hand side of Equation XIII will not have a great bearing upon the unidirectional action of the microphone. I have found that in some cases it is convenient to leave the damping screen 28 out altogether, and when this is done the constants R. and C of the EquationXIII have to be readjusted slightly to compensate for disappearance of the last term. I have found that in a microphone with the casing 23 having a diameter of 6 cm. and the cavity 40 having a volume of 8 cc., the effective distance d is 3.5 cm. and satisfactory operation is obtained when the passage 3| has a circumferential length of 10 cm., a radial length of 0.1 cm. and a thickness of 0.01 cm. These dimensions give an approximate acoustic capacitance C or 5.7X10 cm. per dyne and an approximate acoustic resistance of R of 18 acoustical ohms. Since C, R and d are not calculable with good degree of accuracy in terms of the physical dimensions of the instrument, I'prefer to calculate the approximate dimensions for these terms, and obtain the final values by adjusting the thickness of the passage 3| by means of the screw 4| until correct unidirectional action is obtained. Obviously an alternative procedure would be to adjust instead the volume of the cavity 40 or the length of the distance d which could be done by provisions for adjustably altering the size of the case 23.

Instead of obtaining resistance R by means of the passage 3|, it is possible to substitute the cover 29 with a suitable foraminous supporting member such as a wire-screen disc having a. number of thicknesses of cloth or felt or similar porous material attached to it, completely covering the opening 28, as shown more clearly in Fig. 13. The cloth illustrated is designated by the numeral 10. Fig. 13 is similar to Fig. 6 except as to the use of the cloth screen in place of the narrow passage 3|. By a suitable choice of the thickness and porosity of the material employed, the proper value of acoustic impedance may be obtained. Sometimes it is diflicult to select a material having the exact ratio of resistance to inertance specified in Equation Ix' however, it is seen from Equation X that the squared terms are second-order terms in expansion for cosine function, and therefore the exact relationship between .the inertance term L and the resistance and capacitance terms R and C is not a vital one in obtaining the unidirectional operation of the instrument at low frequencies, and reasonable departure therefrom will affect the unidirectional action but slightly. The important adjustment, however, is the one between the terms expressed in Equation XIII.

I have mentioned previously that th Equati-on XII is valid up to frequencies at which (1 is not larger than approximately one-quarter wavelength of sound. This corresponds, for the instrument of Fig. 6, to a frequency of approximately 2500 cycles per second. It should not be assumed, however, that above' said frequency the unidirectional action ceases, because above 2500 cycles per second, the instrument [tends to become highly unidirectional in favor of sounds arriving from the front because of diffraction and the so-called baille-efi'ect due to the size of the cas 2.3. The unidirectional action is therefore obtained essentially throughout all of the impor-' tant frequency range.

I have found that when a plane wave of constant intensity and varying frequency is impressed upon the front side of the instrument of Fig. 6, the resulting alternating force upon the crystal 24 is approximately proportional to frequency up to the frequency at which one-quarter Wavelength equals the effective distance d, becoming approximately independent of frequency for frequencies at which one-quarter wavelength is larger than the effective distance (1. Since the voltage developed in a crystal is proportional to the force applied, I found ituseful to provide a compensating electrical network which would deliver an output voltage substantially in dependent of frequency at the receiving apparatus. This network consists of a parallel combination of a condenser and a resistance M, both in series with a larger condenser 46; said network is connected across the terminals 41 and 48 .of the crystal, the receiver 49 being connected across the larger condenser.

Fig. 9 shows the frequency response obtained with this microphone and electrical network for plane wave incident upon the front (upper curve) and the rear (lower curve) of the instrument, indicating the type of discrimination obtained at all frequencies. The polar directiv'ity pattern is a cardloid shown in solid lines in Fig. 5.

It may be found convenient in many instances to provide the desired electrical compensation in the receiver 48. For applications in which it is desired togive predominance to higher frequencies of sound, the compensating network may be entirely dispensed with.

Another embodiment of my invention is shown in the part sectional elevation in Fig. 10 and front elevation in Fig. 11. A moving coil consisting of a circular bobbin 50 'having a winding 5| and a dome-shaped diaphragm or cover 62 is arranged to move in an air gap 53 of a magnetic structure, thereby transforming its mechanical motions into electrical energy which is received from the winding Si by means of conductors li The magnetic structure consistsof a cylindrical permanent magnet ll -pmvided at one pole with an internal circular polepiece ll, and having an external pole piece "connected to the other pole of the magnet by means of several connecting rods I! which provide enough cross sectional area .to conduct themagnetic flux.

. uom-,m of the microphone in the middle and do not appreciably interfere .at the same I ll approximately 3.8 cm. square-and a, moving coil approximately 2.8.cm. in diameter, the equiv-' alent front-to-back distance d is approximately pension 8| which permits axialmotions in the;

ai-r gap. The external diameter-of the inner polepiece 5! is slightly smaller thanthe, internal di ameter of the bobbin, [thus forming a narrow pasrange of audio frequencies. I have fond that by subdividing the compliance element into two approximately equal parts shown as Grand Cs interconnected by a series impedance having a resistance value R0 approximately three times the value of R. and an .inertance value In approximately qual-to L. the nnidirectivity Equation VI is very closely satisfied throughoutsubstantially all of the audio-frequency range.

I have-found that in a microphone asv illustrated in Fig. lohaving an external-pole piece 2.5 cm., .and witha total volume of cavities-ll and 63' are cc. correctoperation is obtained if the passage I! has an axiallength of 0.16 cm. and a thickness of 0.013 cm., and the passage II has a circumierenti'allength ot 4.5 cm., a radial length of 0.16 cm. and a thicknessof: 0.007 cm. These sage 2 leading into the cavity 83 which-is defined by.:the moving coil. The stillness of the suspension I is low so that the coilassembly is resonated at a low frequency, preferably in the neighborhood of 60 cycles per second. The resonant effect is not very pronounced. however, because of the damping resulting from motionof air in the passage 82.

The equivalent electrical circuit of the instrument placed in a sound wave is given in Fig. 12. E is the equivalent of sound pressure upon the front of the diaphragm 52. E is the equ valent of the pressure of the sound wave atthe passage 62; Z: is acoutical impedance of the coil and its suspension; R and L, ,the resistance andinert- 1 ance terms of the passage 62; C; isthe acoustic compliance of the cavity 83; Rb and La, the acoustic impedance of the passage 6|; Cb, the acous-v tical compliance of the chamber '0; as in the preceding embodiment. the reactions of the medium are neglected. The eflective acoustical distance between the pressures E and E will be called d, and for frequencies at which this distance is less than a quarter wavelength of sound, E andE may be considered equal in magnitude and displaced by an angle given in Equation I.

Comparing structures illustrated in Fig. 6 and Fig. 10, it will be noticed that the latter is similar to the former with the screen 28 removed, since the impedance of the narrow passage 3! in Fig. 6 corresponds to that of the passage 62 in Fig. 10, and the compliance of the cavity 4. in Fig. 6 corresponds to the total compliance of the cavities SI and 63 in Fig. 10. This similarity may be further seen by comparing the equivalent circuits of Fig. 8 and Fig. 12, the former with the impedances R, L. and C (corresponding to the damping screen '28 and cavity 21 in Fig. 6) removed. Comparing these equivalent circuits, the series impedances R and L of Fig- 8 correspond with the impedances of the same calling in Fig.

12, and the capacity C of Fig. 8 corresponds to thetotal ca acity of the condensers C. and Cs in Fig. 12. Therefore Equation XIII may be used to determine the correct-proportion between the resistance and compliance units in the structure of Fig. 10, the last term of the right hand mem-' ber of the equation being set equal to zero since the front screen is not used in this embodiment. However, in the moving coil structure of Fig. 10, it is not feasible to make the passage 2 narrow enough to obtain the correct ratio between resistance and inertance as expressed in Equation 1x and this would affect adversely the unidirecdimensions giveyan approximate acoustic value of Ca. and Cs ofczzl-x 10 cm. perdyne each, R and Rh of17.3 and 50" acousticalohms. respectively, L and Lb of ;00l5gram=per cm. .These' terms are not calulable with high'degreeof ac curacy and minor adiustmentsa-are required to obtain satisfactory-operation which. is similar to that indicated byrthe performance curves of Fig. 9. It will .be understood that-through judicious application of previously given theory and equations, the above dimensions may be considerably altered without departing from the scope of my invention. 7

A still further u w t'of mar-invention employing a moving conductor as a transducing element is'shown in front elevation in Fig. 14. The voltage generating element is a light metallic conductor of ribbon III which may be corrugated to increase its flexibility, supported at its two ends on insulating supporting members I01 and adapted to move between pole pieces ill of a magnetic structure and convert these motions into'electrical energy which is received by means of conductors H2 and H3. The ribbon is almost as wide as the space between the pole pieces, being separated therefrom ,only enough to move freely therebetween. Figs. 15 andl6 which are cross sectional views of the instrument along the lines I! and it in Fig. 14, respectively, show that the pole pieces Ill and the ribbon ill form a cavity III which is inclosed at the rear by a plate I suitably attached to the support ing member I" by means of screws III. The cavity III is in communication with the exterior by means of passages I formed between the pole pieces llll and the 'back plate Ill, the width of said passages being determined by suitably adlusting the position of the supporting members ll'l behind thepole pieces ill. The passa'gesili have such proportions as to constitute essentially anacoustical mass element. The cavity II. is

provided with a parallel dissipative element.

formed by a pipe or conduit ll! of suitable cross' section and length fitted at the near end into the back plate I and closed at the far end. The conduit is filled with dissipative material ill,

such as loosely packed felt, wool, or cotton, which is retained at the near end with a wire screen .I II or similar foraminous retaining member. It will be evident, however, that other suitable means may be used to produce the parallel resistance effect. The sound pressure at the front or exposed part of the ribbon will be called P and that at the entrance of the passage I" will be called P.

The effective acoustical distance between these pressures will be called d.

Theequivalent electrical circuit of this microphone is shown in" l?ig. 17. Hand 1: represent the sound pressures P and P' respectively, Is represents the acoustical impedance of the ribbon which is assumed'to be a mass, and e isthe force developed across the ribbon due to I and I, L1 is the massof the passage ill, Cs is the compliance of the cavity I00, and R2: is the resistance xvm and, in addition,

d L, if? (XIX) I have found that in a microphone of this description having anequivalent front-to-back distance d of 2 cm. satisfactory operation is obtained if the volume of the cavity I" is 1.2 cm., total length of the slit III is cm., its transverse width 0.8 cm., and its thickness 0.06 cm., and the'conduit ill has an approximate cross sectional area of 1.25 sq cm. and a length of several feet, being filled with loosely packed lambs wool. This corresponds to the following constants in the equivalent circuit of Fig. 1'1: capacitance Cs, 035x10 microfarads; inductance L1, .002 henry; resistance R134 ohms. These constants may be considerably altered within the range of validity of Equations XVIII and XIX without departing from the spirit of my invention.

I claim: a

1. In a unidirectional electroacoustic transducer operating in a medium, means having two pressure-sensitive surfaces adapted to vibrate and translate its vibrations into electrical energy. one of said surfaces being substantially exposed to the medium, a structure forming a cavity in conjunction with the second of said surfaces. a passage defining acoustical resistance and inertance establishing communication between said cavity and the medium and located at such distance from said exposed surface that the ratio of said distance to the wave velocity in the medium substantially equals the product of acoustical compliance of said-cavity and resistance of said passage in any consistent system of units.

2. In-an electroacoustic transducer operating in an elastic medium, means having pressure-sensitive sides adapted to vibrate and translate its vibrations into electrical energy, a structure including phase-shifting acoustical network elements providing unequal access of the sound pressures to said sides and establishing cooperatively with said means an equivalent acoustical distance, one of said sound pressures undergoing a phase shift, due to the action of said network, substantially equal to the angle between the pressures in a wave traveling said distance in the medium itself throughout a range of frequencies.

3. ma microphone, means adapted to be acted upon by direct sound waves and by waves the.

phase of which has been shifted, an acoustical phase-shiftingnetwork having an inlet spaced from said first-mentioned means, said network shifting the phase of soundv pressure at its inlet through an angle proportional to frequency, 75 variations,

and means cooperatingwitb said first-mentioned means for translating the'efiects of said direct Jr-In a microphone, means having two pressure-sensitive surfaces adapted to vibrate and translate vibrations into electrical ener y. one

of' said surfaces being substantially exposed to the transmitting medium, a structure forming a cavity providing acoustical. compliance in conjunction with the other of said *surfaces and having sound permeable means communicating with the external medium, said second means defining acoustical resistance and inertance, said compliance, resistance and inertance having their relationship in such proportionth'at the ratioof sound pressure in the medium at said passage to the sound pressure developed in said cavity is a vector substantially proportionalto gequency throughout a range of audio frequen- 5. In an electroacoustic transducer, a member having two pressure sensitive surfaces adapted to vibrate and translate vibrations, into electrical energy, one of said surfaces being substantially exposed to the medium, a structure forming a cavity in conjunction with the other of saidsurfaces, and a sound permeable means defining acoustical resistance and reactance establishing' communication between said cavity and the medium at' an effective acoustical distance from said exposed surface of approximately one-quarter wavelength of a frequency within the upper range to be translated, the acoustical resistance and reactance of said sound permeable means being so selected in respect to the volume of said cavity and said acoustical distance that the transducer is notably more sensitive in the direction facing said-exposed surface than in all other directions.

6. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, a casing supporting said body and having a cavity, said body forming one of the walls of the cavity, and communieating means between said cavity and the atmosphere, said means defining principally acoustical resistance and adapted in conjunction with said. cavity to shift the phase of sound at all wavelengths substantially larger than the dimensions of said casing.

'1. In a microphone, a moving body adapted to vibrate and translate its vibrations into electrical variations, a casing supporting said body and having a cavity, said body forming one of the walls of the cavity, and substantially direct communicating means between said cavity and the atmosphere, said means comprising a supporting member and a porous material defining principally acoustical resistance secured thereon.

8. In a microphone, vibratory means having two pressure-sensitive surfaces and comprising in part a pieso-electric crystal, one of said surfaces being exposed to the atmosphere, a casing supporting said means and having a cavity, 'said means being adjacent to said cavity, and substantially direct communicating means between said cavity and the atmosphere, said communicating means defining acoustical impedance and adapted in conjunction with said cavity'to shift the phase of sound at all wavelengths substantially'greater than the dimensions of said casing.

9. In a transducer, a diaphragm adapted'to vibrate and convert its vibrations into electrical one of the sides of said diaphragm being substantially exposed to the atmosphere, a casing supporting said diaphragm and having a cavity, said diaphragm forming one of the walls of the cavity, and communicating means providing acoustical impedance between said cavity and the atmosphere, said means comprising a passage, dissipative means associated with the passage, and means for adjusting the impedance by variation of the dimensions of the passage.

10. In a transducer, a moving body having two pressure-sensitive sides, one of which is substantially exposed to the atmosphere, adapted to vibrate and convert its vibrations into electrical variations, a casing supporting said body and having a cavity, and substantially direct communicating means providing acoustical impedance between said cavity and the atmosphere,

, said means comprising a passage, dissipative means associated with said passage, and means for adjusting said impedance.

11. .A sound translating device comprising vibratory means having two pressure-sensitive surfaces and adapted to translate its motions into electrical energy, a casing supporting said means and having a cavity in conjunction with one of thesurfaces of said vibratory means, a passage comprising acoustical resistance and intertance leading into a second cavity and substantially direct communicating means between said first cav-' ity and the atmosphere, said communicating means defining acoustical resistance and inertance, said passages being adapted in conjunction with said cavities to shift the phase of sound pressure at all wavelengths of sound substantially larger than the dimensions of said casing.

12. A sound translating device comprising vibratory means adapted to translate its axial motions into electrical energy and having two pressure-sensitive surfaces, a casing supporting said vibratory means and having a cavity, one of the surfaces of said vibratory means being enclosed by said cavity and the other surfaces being substantially exposed to the atmosphere, a passage comprising acoustical impedance leading from said first cavity into a second cavity and substantially direct communicating means between said first cavityand the atmosphere, said means defining acoustical impedance, the axially projected area of the exposed surface of said vibratory means being substantially equal to the axially projected area of the enclosed surface of said means;

, 13. In a method of obtaining unidirectional operation in a single-unit electroacoustic transducer having two pressure-sensitive surfaces and an enclosure incorporating adjustable sound permeable means defining acoustical impedance and enclosing one of said surfaces, the step of adjustably varying said sound permeable means to the impedance value at which said transducer is most sensitive to sounds arriving from one direction.

14. In a method of obtaining unidirectional operation in a single unit electroacoustic transducer having two pressure-sensitive surfaces and an enclosure incorporating an outlet to sound adapted to support an impedance forming means of porous material associated with said outlet, the step of selecting said impedance material providing animpedance at whichsaid transducer is most sensitive to sounds arriving from one direction.

15. In a microphone, vibratory means having two pressure-sensitive surfaces and comprising in part a piezo-electric crystal, one of said surfaces being substantially exposed to the atmosphere, a casing supporting said means and having a cavity, the vibratory means being adjacent to said cavity, and substantially direct communicating means comprising a passage having acoustical impedance connecting said cavity and the atmosphere, and means for adjusting said impedance.

16. In a microphone, a diaphragm, a coil attached thereto, a magnetic structure having an air-gap in which said coil may vibrate, said diaphragm partially enclosing a. cavity interconnected with a second cavity, said first cavity having a substantially direct passage to the atmosphere, the outlet of said passage being located at a distance from said diaphragm of from 1 to 5 cm.

1'7. In a sound translating device, means having two pressure-sensitive surfaces adapted to vibrate and translate its vibrations into electrical energy one of said surfaces being exposed to the atmosphere, a casing supporting said means and having a cavity, said means being adjacent to said cavity, and substantially direct communicating means between said cavity and the atmosphere, said communicating means defining acoustical impedance and adapted in conjunction with said cavity to shift the phase of sound at all wavelengths substantially greater than the dimensions of said casing,

18. In a. microphone, vibratory means having two pressure-sensitive surfaces and comprising in part a piezoelectric crystal, one of said surfaces being substantially exposed to the atmosphere, a casing supporting said means and having a cavity, said means being adjacent to said cavity, and substantially direct communicating means between said cavity and the atmosphere, said communicating means comprising a supporting member and a porous material defining principally acoustical resistance secured thereon and. adapted in conjunction with said cavity to shift the phase of sound at all wavelengths substantially greater than the dimensions of said casing.

BENJAMIN BAUMZWEIGER. 

